1. Field of the Invention
The present invention broadly relates to a carbon fiber and, more particularly, to a high strength, high modulus pitch-based carbon fiber suitable for use as a reinforcing fiber for light-weight structural material in various industrial fields such as space, automotive and architectural industries.
2. Description of the Related Art
Hitherto, PAN-based carbon fibers have been manufactured and used widely amongst various types of carbon fibers or graphite fibers. In general, PAN-based carbon fibers exhibit superior characteristics, in particular high tensile strength, as compared with pitchbased carbon fibers and, therefore, are used as high strength carbon fibers in various fields. Unfortunately, however, PAN-based carbon fibers show a rather low elastic modulus, e.g., 290 GPa, though some of this type of fibers have very high tensile strength of 5.6 GPa. This is attributable to a fact that high level of elastic modulus can hardly be attained with this type of carbon fibers due to the presence of a practical limit in the crystallization, i.e., degree of graphitization, because of inferior graphitability of this type of carbon fibers. In addition, PAN-based carbon fibers have drawbacks such as high material costs, and are not preferred from the view points of carbonization yield and economy.
Under these circumstances, methods have been proposed for producing pitch-based carbon fibers and graphite fibers which have superior tensile strength and tensile elastic modulus from pitch which is inexpensive.
For instance, Japanese Patent Application KOKOKU No. 60-4286 (U.S. Pat. 4,005,183) discloses a method which has the steps of heating a pitch at a temperature of 350 to 450.degree. C. until about 40 to 90 wt% of meso-phase is generated, spinning a fiber of a carbonaceous pitch which exhibits non-thixotropic characteristic and a viscosity of 10 to 200 poise at the spinning temperature, infusiblizing the spun fiber in an oxygen-containing atmosphere at a temperature of 250 to 400.degree. C., heating the infusiblized fiber to a temperature not lower than 1000.degree. C. in an inert gas atmosphere, and further heating the fiber to a temperature not lower than 2500.degree. C., whereby a graphite fiber is produced which exhibits presence of the (112) cross-lattice line and resolution of the (100) and (101) diffraction lines, which indicate the three-dimensional order of the crystallite of the fiber, and which has an interlayer spacing (doo.sub.2) of 3.37.ANG. or less and a stack height (Lc) of 1000.ANG. or greater.
The graphite fiber heated to 2800.degree. C. as disclosed in the above-mentioned publication shows a tensile strength of about 1.7 to 2.4 GPa (about 250.times.10.sup.3 to 350.times.10.sup.3 psi) and a tensile elastic modulus of about 520 to 830 GPa about 75.times.10.sup.6 to 120.times.10.sup.6 psi).
On the other hand, Japanese Patent Application KOKAI No. 62-104927 (U.S. Pat. 4,775,589) teaches that a pitch-based carbon fiber, which has an orientation angle (.PHI.) smaller than 10.degree., a stack height (Lc) of 180 to 250.ANG., and an interlayer spacing (doo.sub.2) of 3.38 to 3.45.ANG., can be formed from a coal-tar pitch. This pitch-based carbon fiber, however, exhibits a small elongation of 0.38 to 0.43%, though it provides a tensile strength of 2.6 to 3.3 GPa (265 to 333 Kg/mmz) and a tensile elastic modulus of 608 to 853 GPa (62 to 87 ton/mm.sup.2).
Furthermore, Japanese Patent Application KOKAI No. 61-83319 discloses a pitch-based carbon fiber produced from naphthalene through a heat-treatment at a temperature of 2000.degree. C. or higher, the carbon fiber having an orientation angle (.PHI.) smaller than 30.degree. , preferably 15 to 25.degree., a stack height (Lc) greater than 80A but not greater than 200.ANG., preferably 90 to 170A, and an interlayer spacing (doo.sub.2) of 3.371 to 3.440.ANG..
This pitch-based carbon fiber exhibits a tensile strength of 3.1 to 3.9 GPa (318 to 394 Kg/mm.sup.2), a tensile elastic modulus of 234 to 412 GPa (23900 to 42000 Kg/mm.sup.2) and an elongation of 0.9 to 1.4%. In addition, the production cost is high due to the use of naphthalene which is expensive.
Thus, the conventional pitch-based carbon fibers, as can be understood from the above, are inferior at least in elongation and, hence, are difficult to handle. This poses a problem particularly in the production of composite materials.
It is true that the above-mentioned pitch-based carbon fiber produced from naphthalene exhibits a considerably large elongation. This carbon fiber, however, is disadvantageous in that the tensile elastic modulus is small and in that the material cost is high.